Abstract

Quasielastic light scattering spectroscopy has long been used to probe the dynamics of thermal diffusion, sound propagation, and structural relaxation in condensed matter. Ordinarily the region around the zero frequency shift in such spectra is dominated by the Rayleigh line due to temperature/entropy fluctuations. Some materials, however, show an additional central peak attributed to structural or molecular relaxation processes.^1,2 In a heterodyne detected backscattering experiment in cooled fused silica optical fiber, we have resolved a new relaxational light scattering component with a power-law spectrum and an unusual temperature dependence indicative of thermally activated kinetics. Previously reported measurements of light scattering in glass have had neither the sensitivity nor the resolution near 0 Hz to see either Rayleigh scattering or to resolve this new narrow central peak.² At room temperature, the low-frequency light scattering spectrum is consistent with that predicted for conventional Rayleigh scattering. At lower temperatures, where the predicted Rayleigh scattering spectrum should decrease, we find that the light scattering actually rises. We interpreted these data using a modification of a model based on thermally activated kinetics in a system with a distribution of relaxation time constants.³ We are able to define the distribution of barrier heights for a local rearrangement of the fused silica matrix.

© 1988 Optical Society of America